1. Field of the Invention
The invention relates broadly to foundry binders based on organic resins and specifically to resinous binder compositions that are formed by the reaction of phenol formaldehyde condensates and polyisocyanates, known in the foundry trade as phenolic urethanes.
2. Description of the Prior Art
Phenolic urethanes are widely described in the prior art and have been in use for several decades as foundry core and binders. Sand binders form molds or cores for casting of metals, especially aluminum and other lightweight metals which are cast at relatively low temperature. For example, phenolic urethanes based on high molecular weight phenolic resins (i.e., an average of at least 3 aromatic rings per molecule) are described in U.S. Pat. Nos. 3,409,579 and 3,676,392 to Robins. Phenolic urethane foundry binders of a lower molecular weight type are described in U.S. Pat. Nos. 4,148,777 to LaBar et al. and 4,311,631 to Myers et al. While the binders described by these prior patents have, in general, been successful with respect to casting ferrous based metals, which are cast at relatively high temperatures, problems have been observed in using the same binder systems in the casting of aluminum with respect to the breakdown and shake out of the cores after solidification of the metals (also called Thermal Sand Removal, or TSR).
In order to provide a core or mold that is strong enough to maintain its shape and surface during the casting of metals, a fairly high level of binder is required. This is true not only with the phenolic urethane binders but the alkyd-oil binders, the polyester polyol binders, and other types of binders known in the field. However, when sufficient binder is mixed with the sand to form cores and molds having adequate strength to permit handling of the cores or molds and adequate abrasion resistance and hot strength, the resulting cores and molds are difficult to break down and it is difficult to remove the sand from the metal casting, particularly when the casting is made at the relatively low casting temperatures of the light metals, such as aluminum.
Because of the practical importance of this problem to the practices of the foundry art, several approaches have been used in the past to eliminate this problem. On the one hand, organic additives such as sugars have been incorporated into the sand mix. On the other, the quantity of binder has been reduced. Binders with inherently less strength or low heat resistance have also been used. Some have incorporated organic peroxides in the binder in order to aid the oxidative degradation of the said binder at high temperatures; others have included inorganic peroxides in their sand mix. While these efforts have been, by and large, successful in reducing the shake-out problem, there have been highly undesirable aspects associated with them obvious to those familiar wit the art. Therefore, the foundry art has been seeking to find a binder system which will produce cores and molds having adequate strengths and abrasions resistance, but which breaks down well at the casting temperatures of aluminum and magnesium to provide easy shake out.